I think this topic is probably past its sell by date but here’s my 2 cents worth anyway. Looking at the picture of the valve assembly in the Amazon link the OP posted, it will probably be a lot simpler to just unscrew the DC solenoid and screw in a regular 24V AC device. Even if the OP has to buy a cheap complete valve in Home Depot, they only cost $12.
Another solution would be to run the OSPi on its usual AC and then put a 2 Amp bridge rectifier on the output. I bought 10 of these on ebay for $3.95 here http://www.ebay.com/itm/251555357618?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649. This would allow the OP to use a DC solenoid. Obviously he would need to ensure that the DC voltage was correct since 24VAC will produce in excess of 36VDC.